It is essential that ancillary equipment, such as for example, safety equipment such as life jackets, life buoys, life rafts and emergency position indicating radio beacons ("EPIRB"), are rapidly released when an object, such as a ship or aircraft sinks under water. During normal use however, the mechanism which retains an ancillary unit with an object, must be sufficiently strong and efficiently reliable in order that the ancillary unit will only be disengaged under the appropriate emergency conditions.
Typically, mechanisms for retaining and deploying ancillary units at sea have comprised a link which is secured at one part to the ancillary unit, and at another part to a release device which is associated with an object. Upon submergence, the release device triggers the release of the unit, for example by cutting the link connecting the device and the unit.
Hydrostatic release units are commonly used to trigger the release of safety equipment. Hydrostatic release units respond to the change in ambient pressure that occurs when they become submerged under water. Each unit has a pressure-sensing means, usually a pressure-sensitive diaphragm, that reacts to the increase in pressure and triggers the release of the ancillary unit.
EP-A-0676 327, for example, describes a hydrostatic release unit wherein a change in ambient pressure opens a liquid-filled chamber, thereby releasing a hydrostatic pressure behind a reciprocable member; as a result, the reciprocable member facilitates the release of an ancillary unit
GB-A-2,300,874 describes a release unit wherein an increase in pressure triggers a pressure-sensitive spring mechanism to open a latch and release a life raft.
Similarly, U.S. Pat. No. 5,365,873 describes a mechanism wherein an increase in pressure results in the displacement of a slider which releases a life-raft or the like.
Another known hydrostatic release unit comprises a pressure-sensitive diaphragm which triggers a spring-loaded block. On release of the spring at a predetermined pressure, a blade cuts a rope which links the ancillary unit to the release device and the ancillary unit is then free to float to the surface. In an alternative mechanism, depression of a pressure-sensitive diaphragm allows water to flood into a compartment containing a soluble tablet which retains a releasing means, such as a blade for example, in position. Once the tablet has dissolved, the blade is released to sever the rope.
Most of the hydrostatic release units presently available, however, suffer from a number of disadvantages. The use of a soluble tablet for instance, can be unreliable and prone to accidental release under the typically moist and damp conditions that inevitably co-exist with seafaring vessels. Spring mechanisms also suffer detriment under moist conditions which may affect their mechanical operation. Moreover, larger and more powerful springs are needed in order to generate the force required to cut through increasingly thick linking ropes or ropes made from tougher material and the size of a hydrostatic release unit must be increased accordingly to accommodate the necessary increase in power required. This is increasingly impractical in light of the small size of life buoys and EPIRBs that are currently available.